Role of N-Propyl-N-Methyl Pyrrolidinium bis(trifluoromethanesulfonyl)imide as an Electrolyte Additive in Sodium Battery Electrochemistry

Electrolyte compatibility with sodium metal and known sodium-ion battery cathodes is of intense interest recently. In this work, electrolytes including an ionic liquid (IL) additive are formulated and shown to enhance the properties of the well-known Na₃V₂(PO₄)₃/C (NVP/C) cathode. NVP/C composite material was synthesized by a carbothermal reduction process and studied in cells containing either organic electrolyte (1 M NaClO₄ in PC) or organic electrolyte with added [C₃mpyr][NTf₂] IL. In an electrolyte containing 5 wt % [C₃mpyr][NTf₂] IL, the cathode material exhibited a stable capacity of 107.0±0.7 mAh g⁻¹ after 50 cycles at a C/2 rate; in contrast, the capacity obtained with the organic electrolyte was only 85±1 mAh g⁻¹ after 50 similar cycles. In situ impedance spectroscopy revealed that the electrode-electrolyte charge-transfer resistance is lower in the organic electrolyte with IL. Consistent with this finding, XPS, SEM, EDX and TEM results indicate that the interfacial layer formed on the surface of the NVP/C cathode material in the presence of the IL is more compact than that formed in the organic electrolyte, and that reaction between cathode and electrolyte is suppressed by the IL.